I don’t know if I can give you a bright yellow line number of where the cutoff should be. I think that number has to be set based on just how much different two teams prestige levels should be and that still allows the lower prestige team to “have a chance” to sign the player given that everything else, except prestige, is equal.
I assume, perhaps falsely, that HD had something in mind when they designed the system and only they have the data to see if it’s “working as intended”.